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I think I need a SPST 12V 30 Amp Latching relay.

I would like to fit the circuit to my van for a machine which cannot be run whilst the vehicle is in motion, therefore I will use a micro switch with the clutch pedal, therefore when the clutch pedal is pressed the machine does not work. I will be wiring the micro switch output in to the relay (coil side), then put a negative to the other side of the coil. Then I will have a common from the battery to the relay. Then I will have the output of the relay to the machines on/off switch.

Now my problem is that I would also like a momentary switch in this circuit somewhere, so that even if the clutch is not pressed the machine is not on. I cannot use the normal machine off off switch as it is too far in the back of the van. Where would I put this switch and which exact relay do I need for this setup.

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  • \$\begingroup\$ Triggering from the brake light circuit would allow minimum effort. Stand on brake and machine shops. \$\endgroup\$
    – Russell McMahon
    Sep 10 at 8:34
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perhaps something like this:

schematic

simulate this circuit – Schematic created using CircuitLab improved version:

schematic

simulate this circuit

The relay is just an ordinary automotive headlamp/horn relay the 30A fuse prevents damage to the cable to the machine from starting a fire and the 1A fuses do the same for the control circuit (allowing the use of smaller wire for those circuits)

pressing the stop button or the clutch cuts the power to the relay turning it off.

pressing the start button feeds power to the relay turning it on and then the relay feeds power to itself after that.

the 1A fuses can be 3AG fuses in in-line fuse-holders the 30A should probably be an automotive blade fuse in a suitable fuse block.

lever of the clutch pedal may suffice.

schematic

simulate this circuit

finally here's a simplified version. the start buttone will need to be a heavy duty type capable of running the machine.

the switch on the clutc need to be heavy duty, and as it is a switch to ground a simple contact that contacts the

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  • \$\begingroup\$ Thanks for that, what would this relay be called, also I see there is a diode in this circuit, would this be built into the relay or is this a separate item I would need. Thank you very much. \$\endgroup\$
    – Abcd123
    Jun 16 '19 at 1:54
  • \$\begingroup\$ that's a separate item it prevents the start button form feeding power directly to the machine so that the safety switchis always work. relay would be called "30A horn relay" or "40A horn relay" would work too. should cost less than $10, some are availave with a build-in fuse slot that could work instead of the separate 30A fuse. \$\endgroup\$
    – Jasen
    Jun 16 '19 at 2:07
  • \$\begingroup\$ Jasen just add another diode reversed across coil. The machine will also be inductive so some derating is needed and possibly a snubber cct. Then relocate Relay close to machine Otherwise good job to make a non-latching relay to latch. \$\endgroup\$ Jun 16 '19 at 2:23
  • \$\begingroup\$ close to battery or close to machine makes little difference as they will be at opposite ends of the vehicle, most of the controls are at the front, relay diode not needed as no electronics is involved. automotive is a hars environment the rules are different \$\endgroup\$
    – Jasen
    Jun 16 '19 at 2:37
  • \$\begingroup\$ You're right, but 1kV spikes will still be generated on DC power line throughout wiring across the open switch and past user peripherals nearby so a bidirectional TVS may prevent surprises. littelfuse.com/products/tvs-diodes/… for both reactors \$\endgroup\$ Jun 16 '19 at 2:48

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